Typical alloy proportions are 90:10 or 70:30 (Pt:Ir). These have the chemical stability of platinum, but increased hardness. The Vickers hardness of pure platinum is 56 HV while platinum with 50% of iridium can reach over 500 HV. This improved hardness has also been considered as beneficial for use in platinum jewellery, particularly watch cases.
Their well-known use is in metrology, where they are used to make the international prototype measures used by international standards bodies for mass standards such as the international prototype kilogram and the international prototype metre, although the latter has now been superseded by a standard based on the vacuum speed of light.
The other extremely widespread use for Pt/Ir alloy is fabrication of metal microelectrodes for electrical stimulation of nervous tissue and electrophysiological recordings. Pt/Ir alloy has an optimal combination of mechanical and electrochemical properties for this application. Pure iridium is very difficult to pull into small diameter wires; at the same time, platinum has a low Young's modulus which makes pure platinum wires bend too easily during insertion into nervous tissue. Additionally, platinum-iridium alloys containing oxides of both metals can be electro-deposited onto the surface of microelectrodes.
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